Mobile Robot in gazebo using URDF files instead of SDF, the wheels are disappearing

I am trying to create a mobile robot using urdf and following the tutorial "Gazebo : Tutorial : Make a Mobile Robot" that creates SDF file. In my case I changed the size of the robot and I am creating it using URDF. I did the following:
1- I created a file called mobileRobot.urdf.xacro